This change makes existing Lookup API test cases conform to the new
return value form that all the Lookup APIs except LookupTXT must return
a single or multiple absolute domain names.

Updates #12189.
Fixes #12193.

Change-Id: I03ca09be5bff80e818fbcdc26039daa33d5440a8
Reviewed-on: https://go-review.googlesource.com/17411
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>